FACE MASK PENALTY

WHY ARE RUNNING BACKS AND RECEIVERS ALLOWED TO STIFF ARM OR EVEN GRAB FACE MASK OF DEFENSIVE PLAYERS WITHOUT PENATY? THEY COMPLETELY CONTROL THE PLAYER WHEN THEY DO THAT. THIS CERTAINLY NEEDS TO BE ADDRESSED IN RULES MEETINGS. IT IS VERY UNFAIR TO THE DEFENSE. ANY WHERE ELSE IT IS A PENALTY, WHY NOT WHEN A RUNNIG BACK OR RECEIVER DOES IT??